Aluminum base alloy



United States Patent 3,304,209 ALUMINUM BASE ALLOY William A. Anderson,Verona, and William D. Vernam,

New Kensington, Pa., assignors to Aluminum Company of America,Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Filed Feb. 3,1966, Ser. No. 524,822 4 Claims. (Cl. 14832.5)

This application is a continuation-in-part of our 00- pending patentapplication Serial No. 304,677, filed August 26, 1963, and nowabandoned.

This invention relates to aluminum base alloys suitable for makingwrought structural members, and it is more particularly concerned withthose alloys which contain zinc and magnesium as the principal addedelements in respect to the amounts present in the alloys.

Aluminum base alloys of the type just referred to have been known formany years. When worked, solution heat treated and age hardened, theydevelop high tensile and yield strengths, especially if a small amountof copper is included in the composition. Although the attainment of ahigh strength is advantageous in many instances, the alloys may bedeficient in other respects, such as resistance to stress corrosion,weldability and notch toughness. We have discovered that withinrelatively narrow limits zinc, magnesium and other elements unite toproduce an alloy which, in the absence of copper (except as animpurity), possesses a unique combination of properties.

It is one of the objects of our invention to provide a strong aluminumbase alloy which in the age hardened condition is also resistant tostress corrosion cracking. Another object is to provide an alloy whichcan be readily Welded. Still another object is to provide an alloyproduct which is free from segregation of relatively insolubleconstituents. Still another object is to provide an alloy, which,particularly in the solution heat treated and age hardened condition,has a high notch toughness. These and other objects and advantages willbecome apparent from the following description and examples.

We have found that the combination consisting essentially of 3.5 to 8%zinc, 0.75 to 4.3% magnesium, 0.05 to 0.75% manganese, 0.06 to 0.30%chromium, 0.06 to 30% zirconium and 0.01 to 0.15% titanium with aluminumyields an alloy which is free from segregation, if the proper portionsof high melting elements are employed, and which when worked and agehardened has unexpectedly high properties. The foregoing percentagevalues refer to percent by weight of the several elements. Inparticular, the alloys so produced possess a high strength, a highresistance to stress corrosion cracking, a high notch toughness at lowtemperatures and an excellent welda-bility. The combination of theseproperties and their high values which characterize our alloys are notfound in the alloys which are devoid of the high melting point elementsmanganese, chromium, zirconium and titanium.

In respect to impurities, the alloys should be free from copper, thatis, the alloys should contain not more than 0.1% of this element. Ironshould not exceed 0.4% and silicon should not exceed 0.35%, the totaliron plus silicon preferably not exceeding 0.6%.

The wrought products made from the alloys should be free fromsegregation of the relatively insoluble high melting point elementsmanganese, chromium, zirconium and titanium. This means that theseelements and any constituents they form with aluminum or with each otheror both should be in a finely divided condition and be uniformlydispersed, as compared to coarse particles many times the size of thefine particles which may be Patented Feb. 14, 1967 non-uniformlydistributed throughout the alloy products.

Freedom from segregation serves to improve the prop erties referred toabove. To assure such freedom from segregation the high melting pointelements should be employed in amounts within the above-described rangestherefor such that the sum of percent chromium per-cent zirconium+ 1.23

percent titanium-l-O. 19 X percent manganese does not exceed 0.6%, andpreferably does not exceed 0.5%. This means that all of these fourelements cannot be used at one time in the maximum amounts stated above.It will be appreciated that if the alloys are free from segregation intheir initial form, that freedom will persist during subsequent workingoperations and thermal treatments.

The zinc and magnesium components are soluble in solid aluminum and theyare chief contributors to strength when the alloy is age hardened. Ifless than the minimum amounts are employed, the strength suffers whileif the maximum quantities are exceeded, fabrication and corrosionproblems are encountered. The zinc content should, in any case, exceedthe magnesium content of the alloy. For example, these components maydesirably be present in such proportions as 4% zinc and 2% magnesium or7% zinc and 1% magnesium. The term age hardened as used herein refers toboth spontaneous aging at room temperature and to a low temperaturetreatment applied to an alloy where a substantial amount of the zinc andmagnesium is in a state of solid solution before age hardening isstarted, as more particularly described below.

The combination of manganese, chromium, zirconium and titanium in thealloys containing zinc and magnesium within the ranges mentioned abovealso improves the resistance to stress corrosion cracking, notchtoughness, and minimizes the occurrence of cracks Where the alloys arebeing fusion welded. Alloys without the four high melting point elementsdo not have the resistance to corrosion that is generally required nordo they possess high notch toughness, especially at low temperatures.The alloys are especially sensitive to the presence or absence ofzirconium, where the other three elements are present, in respect toimproved weldability, i.e. freedom from cracks in or adjacent to theweld bead, when a filler metal is used which also contain-s zinc andmagnesium as the chief added alloy components. Filler metal compositionsespecially suited for use with the alloys described herein are describedand claimed in the co-pending patent application of Dudas and Collins,Serial No. 306,622. The presence of zirconium in the alloys being weldedis especially effective in eliminating cracks in or adjacent to the welddeposit under those conditions. The amount of zirconium which iseffective for this purpose, as well as the other purposes named above,is small, less than 0.06% failing to have any significant effect whilemore than 0.30% introduces problems of segregation.

The manganese, chromium and titanium components are also important inobtaining a good welded joint and in obtaining improved resistance tocorrosion and stress corrosion cracking.

As mentioned above, wrought alloys of this type develop their higheststrength following a solution heat treatment and age hardening. Solutionheat treatment can be effected by heating at a temperature between 700and 970 F. for a sufiicient length of time to bring about solution ofthe zinc and magnesium, for example, /2 to 24 hours depending upon themass of material being treated, the temperature and other well-knownfactors. To retain a substantial portion of the dissolved elements in astate of solution the alloys are cooled to room temperature or slightlyabove that temperature by quenching in a suitable medium. The alloys canbe cooled relatively rapidly or slowly and for this reason they areconsidered to have a low quench sensitivity which is an importantoperational advantage.

The resistance to stress corrosion cracking is, however, affected by thethermal treatments, the maximum resistance being achieved through asequence of thermal treatment steps being achieved through a sequence ofthermal treatment steps as described and claimed in our copending patentapplication Serial No. 276,156. Where structures made from these alloysare not exposed to stress corroding conditions, or the conditions arerelatively mild, the alloy products can be given a conventional singlestep age hardening treatment. The artificial age hardening treatment inany case consists of heating the alloys containing the zinc andmagnesium in solid solution to a temperature between 200 and 320 F. andholding them within that range for a total period of to 48 hourswhereupon the alloys are cooled to room temperature. However, if thewrought products of these alloys have received a solution heat treatmentand are quenched, they may be allowed to age harden at room temperature.Al-

measure of resistance to impact. The notch toughness of our alloys inthe age hardened condition is significantly higher than that of some ofthe present commercial high strength alloys, particularly at sub-zerotemperatures. The test used to determine notch toughness is described inthe Bulletin of the American Society for Testing Materials for January1960, pages 29 to 40 and February 1960, pages 17 to 28. The tensilestrength is determined on notched specimens at the desired temperatureand the values compared to those of unnotched specimens, the resultbeing expressed in a ratio of the former to that of the latter.

The alloys which have been described can be melted and cast according tonormal practices. For those that are to be made into wrought productsany of the conventional metal working processes can be used such asrolling, forging, extrusion, drawing and the like.

The advantages of our invention are illustrated by the following testdata.

The tensile properties and notch toughness of alloys, with and withoutzirconium, are shown from the following tests at room temperature and at320 F. The composition of the alloys is given in Table I below.

TABLE I.PERC-ENT COMPOSITION OF ALLOYS Alloy Percent Percent PercentPercent Percent Percent Percent Percent Percent Zn Mg 1 11 Cr Ti Zr CuFe Si 4. 10 2. 24 0. 22 0.12 0. O1 O. 04 0.17 0. 1O 4. 15 1. 92 0. 22 O.11 0. 01 0. l2 0. 03 0. 18 0. 08 4. 32 2. 51 0. 28 0. 16 0. 02 0. 03 0.l7 0. 08 4. 18 2. 67 0. 26 0. 16 0. 02 0.14 0.03 0. 16 0. 08

though the thermal treatments are important in developing strength,resistance to corrosion and other properties, the presence of therelatively insoluble elements mangan ese, chromium, zirconium andtitanium are also important, for in their absence the desired propertiesare not attained.

In addition to possessing the properties mentioned Alloys A and B werecast and rolled to sheet 0.063 inch in thickness in conventional manner.Samples of the sheet were solution heat treated 1 hour at 860 F.,quenched in cold water and age hardened by heating at 250 F. for 48hours. Tensile properties and notch tensile strength were determined at75 and 320 F. with the following results at a theoretical stressconcentration above the thermally treated alloys are highly resistant tofactor of 17.

TABLE II.TENSILE PROPERTIES AND NOTGH STRENGTH OF SHEET SPECIMENSTensile Properties Ratio Notch Alloy Tcmp., Percent Tensile F. TensileYiclrl Elongation Strength, Notch T.S.l Notch TSJ Strength, Strength,p.s.i. Unnotched Unnntched p.s.i. psi. 'l.S. Y.S.

75 65, 200 58, 600 10. 5 63, 800 0. 98 l. 09 --320 83, 900 70, 400 15. 059, 000 0. 68 O. 81 75 64, 300 58, 890 11. 0 63, 900 0. 99 l. 09 3'2083, S00 70, 100 15. 5 72, 200 0. 86 1. 03

impact such as encountered in ballistic plate. It has been found thatballistic members made of these alloys, when solution heat treated andage hardened, are more resist- :ant to impact than members made of aconventional alloy composed of aluminum, 4.5% magnesium, 0.45% manganeseand 0.1% chromium. It has been observed, for example, that a higherprotection is obtained on a weight basis than with the aforesaidconventional alloy.

A property, known as notch toughness, is another Alloys C and D werecast and rolled to plate 1% inch in thickness, solution heat treated,quenched and aged at the same temperatures and for the same length oftime as the preceding sheet specimens. Transverse specimens were cutfrom the plate and tensile and notch properties determined as above witha theoretical stress concentration factor of 12. The results are givenin Table III.

TABLE III.-TENSILE PROPERTIES AND NOTCH STRENGTH OF PLATE SPECIMENSTensile Properties Rat-i0 KNOtClII Allo Tern Percent ensi e y TensileYield Elongation Strength, Not-ch T.S./ Notch T.S. Strength, Strength,p.s.i. Tensile 'I.S. Tensile Y.S.

p.s.i. p.s.i.

68, 60. 000 13. 0 89, 400 p 1. 31 1. 49 320 86,100 71. 800 13.0 75, 8000. 88 1. 06 75 6'7, 200 59. 100 13.5 90. 800 1.35 l. 54: 32[) 85, 60070, 800 13. O 84, 400 0. 98 1. 19

In the test a filler metal wire was used having the followingcor'nposition: 3.90% magnesium, 2.00% zinc, 0.51% manganese, 0.10%chromium, 0.12% titanium, 0.14% iron and 0.10% silicon, the balancebeing aluing procedures Were followed one in which the fillet was minum.Hot rolled plates were used in the as-rolled condition, the plateshaving the compositions stated in Table V.

TABLE V.PERCENT COMPOSITION OF ALLOYS Alloy Percent Percent PercentPercent Percent Percent Percent Percent Percent Zn Mg Mn Cr Zr i Cu FeSi zirconium is to be seen in the following examples. Two alloys wereemployed which had the composition appearing in Table IV.

TABLE IV.PERCENT COMPOSITION OF ALLOYS Alloy Percent Percent PercentPercent Percent Percent Zn Mg Mn 1 7.1 Ti

The alloys were cast and rolled to sheet 0.064 inch in thicknessaccording to usual practice. Sections cut from the sheet were solutionheat treated 1 hour at 860 F., quenched in cold Water, and age hardened48 hours at 250 F. Test specimens were stressed to 75% of the yieldstrength of the alloys and exposed to the wellknown alternate immersiontest in an aqueous solution of 3 /2% NaCl. All of the specimens of alloyE failed within 33 days whereas all of the specimens of alloy F remainedintact. The improvement gained by the presence of zirconium is clearlyevident.

The elfect of manganese, chromium, zirconium and titanium upon thetendency for weld beads to crack between structural members isillustrated in the following test which involved welding a /2 inch thickplate 10 inches in length to a 1 inch thick plate of the same length inthe form of a T-joint. Welding was performed by the inert gas arcWelding method wherein a filler rod was used to form a fillet on bothsides of the junction of the /2 inch plate with the 1 inch plate. Twoweldformed in a single pass from one end of the plate to the other, anda second one on another specimen wherein the fillet formation wasinterrupted and then continued. Cracks generally occur longitudinally ofthe weld bead and the length of the crack is considered to indicaterelative weldability of the structural or parent metal mem bers.

A pair of plates of alloy G Were welded by each of the two weldingprocedures. Plates of alloy H were welded in the same manner. The lengthof cracks in the Weld beads produced by the two methods was measured andan average value determined. It was found that in the case of alloy Gthe cracks extended over 49% of the total length of the weld beadwhereas in the case of alloy H, the carcks amounted to only 3% of thetotal length of the weld beads. This is considered to show quite clearlythe superior welding characteristics of the alloys of the invention.

Having thus described our invention and certain embodiments thereof, weclaim:

1. A copper-free aluminum base alloy consisting essentially of aluminum,3.5 to 8% zinc, 0.75 to 4.3% magnesium, wherein the zinc content alwaysexceeds the magnesium content, 0.05 to 0.75% manganese, 0.06 to 0.30%chromium, 0.06 to 0.30% zirconium and 0.01 to 0.15% titanium, the ironimpurity not exceeding 0.4% and the silicon impurity not exceeding0.35%, said manganese, chromium, zirconium and titanium being employedin amounts within the foregoing ranges such that the sum of percentchromium-i-percent zirconium-H23 percent titanium+0.19 percent manganesedoes not exceed 0.6%, said alloy being characterized by substantialfreedom from segregation of said manganese, chromium, zirconium andtitanium components.

2. An alloy according to claim 1 wherein the zinc content exceeds themagnesium content and the sum referred to does not exceed 0.5%.

3. An alloy according to claim 1 having an internal structure developedby age hardening at 200 to 320 F. for a total period of 10 to 48 hours.

4. An alloy according to claim 1 having an internal structure resultingfrom a solution heat treatment at 700 to 970 F., and age hardening at200 to 320 F. for a total period of 10 to 48 hours.

References Cited by the Examiner UNITED STATES PATENTS 2,106,827 2/1938Brown 146 X 2,146,330 2/ 1939 Comstock 75146 2,985,530 5/1961 Fetzer etal. 75146 2,993,783 7/ 1961 Martin 75146 3,133,839 5/1964 Thomas 75146FOREIGN PATENTS 656,476 8/ 1951 Great Britain. 932,575 12/1947 France.

DAVID L. RECK, Primary Examiner,

Q. N. LOVELL, Assislan; Examiner,

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 304,209 February 14, 1967 William A. Anderson et a1 It is hereby certifiedthat error appears in the above numbered patent requiring correction andthat the said Letters Patent should read as corrected below.

Column 1, line 45, for "30%" read 0 30% column 2, line 7 for "chromiumread chromium column 6, line 5, strike out "ing procedures were followedone in which the fillet was" and insert the same after "weld-" in line48, column 5; column 6, line 25, for "carcks" read cracks Signed andsealed this 17th day of October 1967 (SEAL) Attest:

EDWARD J. BRENNER Commissioner of Patents Edward M. Fletcher, Jr.

Attesting Officer Dedication 3,30%,209.William A. Anderson, Verona andWflliam D. Vernam, New Kensinglbon, Pa. ALUMINUM BAE ALLOY. Patent datedFeb. 14, 196 Dedication filed Apr. 16, 1970, by the assignee, AluminumCompany of America. Hereb dedicates the entire patent to the Public.

[ 72ml Gazette August 18, 1970.]

1. A COPPER-FREE ALUMINUM BASE ALLOY CONSISTING ESSENTIALLY OF ALUMINUM,3.5 TO 8% ZINC, 0.75 TO 4.3% MAGNESIUM, WHEREIN THE ZINC CONTENT ALWAYSEXCEEDS THE MAGNESIUM CONTENT, 0.05 TO 0.75% MANGANESE, 0.06 TO 0.30%CHROMIUM, 0.06 TO 0.30% ZIRCONIUM AND 0.01 TO 0.15% TITANIUM, THE IRONIMPURITY NOT EXCEEDING 0.4% AND THE SILICON IMPURITY NOT EXCEEDING0.35%, SAID MANGANESE, CHROMIUM, ZIRCONIUM AND TITANIUM BEING EMPLOYEDIN AMOUNTS WITHIN THE FOREGOING RANGES SUCH THAT THE SUM OF PERCENTCHROMIUM + PERCENT ZIRCONIUM + 1.23 X PERCENT TITANIUM + 0.19 X PERCENTMANGANESE DOES NOT EXCEED 0.6%, SAID ALLOY BEING CHARACTERIZED BYSUBSTANTIAL FREEDOM FROM SEGREGATION OF SAID MANGANESE, CHROMIUM,ZIRCONIUM AND TITANIUM COMPONENTS.